It is possible to arrange surface characteristics or markings on a body such that they form a pattern which contains information, such as the identification of the body itself, and so that the pattern can be read with suitable detection equipment. The arrangement of markings or characterizations can be regarded, in a broad sense, as an encoding technique. Thermally readable codes of this type have been readable only with difficulty and with correspondingly expensive apparatus. They have the advantage that they can be arranged so that the coded portions of the surface or body do not differ from the surrounding surface in the visible region of the spectrum and are therefore, for all practical purposes, invisible.
Such surface encoding is thus quite secure against fraud and, because of the expense of the apparatus for reading, can also be regarded as being relatively secure against unauthorized reading. Prior art devices are, however, only manufacturable at considerable expense.
A thermally readable recording technique is shown in U.S. Pat. No. 3,511,973 in which a recording medium, normally having a substantially uniform resistivity, is altered in a pattern of selected, relatively small areas so that those areas have different resistivities. When a current is applied to the medium, the areas of differing resistance exhibit different losses from the remainder of the medium and, therefore, reach different temperatures which can be scanned to determine the pattern using suitable detector means.
This technique is quite limited in its field of use because electrical contacts must be provided on the surface and those contacts must be connected to a current source in order for the temperature differences to be made detectable. The fact that the electrical contacts would likely be visible and conspicuous impedes secret characterization. Because of the necessity of connection to a current source, reading is limited substantially to stationary reading devices. These are significant disadvantages.